On Tue, 3 Jul 2012 11:10:24 +0100 Mel Gorman <mel@xxxxxxxxx> wrote: > > >>>>>+ if (cc->order> 0) > > >>>>>+ zone->compact_cached_free_pfn = high_pfn; > > >>>> > > >>>>Is high_pfn guaranteed to be aligned to pageblock_nr_pages here? I > > >>>>assume so, if lots of code in other places is correct but it's > > >>>>unobvious from reading this function. > > >>> > > >>>Reading the code a few more times, I believe that it is > > >>>indeed aligned to pageblock size. > > >> > > >>I'll slip this into -next for a while. > > >> > > >>--- a/mm/compaction.c~isolate_freepages-check-that-high_pfn-is-aligned-as-expected > > >>+++ a/mm/compaction.c > > >>@@ -456,6 +456,7 @@ static void isolate_freepages(struct zon > > >> } > > >> spin_unlock_irqrestore(&zone->lock, flags); > > >> > > >>+ WARN_ON_ONCE(high_pfn& (pageblock_nr_pages - 1)); > > >> /* > > >> * Record the highest PFN we isolated pages from. When next > > >> * looking for free pages, the search will restart here as > > > > > >I've triggered the following with today's -next: > > > > I've been staring at the migrate code for most of the afternoon, > > and am not sure how this is triggered. > > > > That warning is placed in isolate_freepages(). When the migration > scanner and free scanner have almost met it is possible for high_pfn to > be > > cc->migrate_pfn + pageblock_nr_pages > > and that is not necessarily pageblock aligned. Forcing it to be aligned > raises the possibility that the free scanner moves to another zone. This > is very unlikely but could happen if a high zone was very small. > > I should have caught this when the warning was proposed :( IMO it's > safe to just drop the warning. The rest of this patch takes care to ensure that ->compact_cached_free_pfn is aligned to pageblock_nr_pages. But it now appears that this particular site will violate that. What's up? Do we need to fix this site, or do we remove all that make-compact_cached_free_pfn-aligned code? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>